A major technical problem confronted by coal-fired power-generating installations such as electric utility plants is balancing the flow rates of multiple airstreams containing particulate coal. This problem arises in the design of flow paths between a coal pulverizer and a combustion chamber or a “firebox” having spaced peripheral injection points. The coal laden airstreams tend to follow general hydraulic flow principles in that they tend to choose the path of least resistance. Accordingly, parallel flow paths of different lengths with a common entry will often exhibit substantially different flow rates resulting in different fuel feed rates and an asymmetrical fire ball in the combustion chamber.
The problem is typically confronted by the use of ring-like flow restrictors in the various parallel paths. Such restrictors are placed in the flow paths to reduce the interior diameter in the shorter, less-resistant paths. However, a restrictor size which is correct for one flow rate may not work for a different flow rate. Thus, variable orifice devices such as the gate valve described in the U.S. Pat. No. 6,009,899 have been developed. Gate valves present their own problems; for one, they tend to include structures which can be extended orthogonally across the flow path. As such, they create turbulence and eddy currents and are subject to rapid wear.